1. Field of the Invention
The present invention relates to a hydraulic pressure control apparatus for controlling a fluid pressure in a hydraulically operated brake cylinder.
2. Discussion of the Related Art
JP-A-10-322803 (published on Dec. 4, 1998, before the present invention was made) discloses such a hydraulic pressure control apparatus using an electrically operated hydraulic pressure source as a high-pressure source. This hydraulic pressure control apparatus includes (1) the above-indicated high-pressure source which includes a pump, an elcctrc motor or operating the pump, and an accumulator for storing a working fluid pressurized by the pump, for supplying the pressurized fluid to a brake cylinder, (2) a low-pressure source for storing the fluid discharged from the brake cylinder, (3) a solenoid-operated pressure increase control valve disposed between the high-pressure source and the brake cylinder, and electrically controlled for controlling the pressure of the fluid in the brake cylinder, to a level depending upon an electric energy applied thereto, (4) a solenoid-operated pressure reduction control valve disposed between the low-pressure-source and the brake cylinder, and electrically controlled for controlling the pressure of the fluid in the brake cylinder, to a level depending upon an electric energy applied thereto, and (5) a control device for controlling the pressure increase control valve and the pressure reduction control valve to regulate the pressure of the fluid in the brake cylinder, while controlling the electric motor to hold the pressure of the fluid in the accumulator at a predetermined level.
When the fluid pressure in the brake cylinder is increased under the control of this hydraulic pressure control apparatus, the electric energy to be applied to the solenoid-operated pressure increase control valve is controlled by the hydraulic pressure control apparatus, with the solenoid-operated pressure reduction control valve held in its closed state. The pressure of the fluid to be supplied from the accumulator to the brake cylinder through the pressure increase control valve is controlled according to the controlled electric energy applied to the pressure increase control valve. When the fluid pressure in the brake cylinder is reduced under the control of the hydraulic pressure control apparatus, the electric energy to be applied to the pressure reduction control valve is controlled by the apparatus with the pressure increase control valve held in its closed state. The pressure of the fluid in the brake cylinder is reduced to a level corresponding to the controlled electric energy. Thus, the pressure of the fluid in the brake cylinder is controlled by controlling the electric energies to be applied to the pressure increase control valve and the pressure reduction control valve.
The assignee of the present invention proposed a hydraulic pressure control apparatus as disclosed in Japanese Patent Application No. 10-102476 which has not been laid open at the time the present invention was made). The hydraulic pressure control apparatus disclosed in this Application includes a normally open, solenoid-operated shut-off valve which is disposed between the brake cylinder and a reservoir and which is placed in its open state when an electric energy is not applied thereto and in its closed state when the electric energy is applied thereto. In a conventional hydraulic pressure, a solenoid-operated shut-off valve disposed between the brake cylinder and the reservoir is a normally closed valve which is placed in its closed state when no electric energy is applied thereto, in order to prevent delayed initiation of a braking effect provided by the brake cylinder. After each braking operation of the brake cylinder by releasing of a brake operating member, for instance, this normally closed shut-off valve is held in the open state for a predetermined time which is estimated to be required for substantially zeroing the pressure of the fluid in the brake cylinder. Then, the electric energy is completely removed from the shut-off valve, so that the shut-off valve is returned to the original closed state. However, the brake cylinder may have a residual fluid pressure even after the expiration of the above-indicated predetermined time, that is, even after the pressurized fluid has been discharged for the predetermined time. This residual fluid pressure may cause a so-called “brake drag”, namely, some degree of braking even after the braking operation in question. This brake drag can be avoided where the shut-off valve is a normally open valve which is returned to the open state by removal of an electric energy after each braking operation. On the other hand, however, the normally open shut-off valve must be quickly brought to the closed state when it is required to increase the fluid pressure in the brake cylinder. To this end, the normally open shut-off valve disclosed in the above-identified Patent Application is energized with a maximum electric energy by the pressure control apparatus, when the fluid pressure in the brake cylinder is increased. Accordingly, this normally open shut-off valve requires a comparatively large amount of consumption of the electric energy, and suffers from a relatively large operating noise, a relatively short life, and other problems.